In recent years, a Doherty circuit has been widely used for a transmitting high output amplifier of a mobile communication base station. As advantages of the Doherty circuit, its power efficiency is infinitely larger than that of a single amplifier or a parallel-connected amplifier.
The Doherty circuit has a carrier amplifier and a peak amplifier. The carrier amplifier is biased in class A or AB mode, and the peak amplifier is biased in class C mode (see, for example, Japanese Laid-open Patent publications No. 2007-134977, 2008-147857, and 2008-22513, and Published Japanese Translation of a PCT Application No. 2000-513535). Input signals are distributed into two, and one signal is given to the carrier amplifier and the other signal is given to the peak amplifier via a λ/4 line.
In the Doherty circuit, when an input signal is small, only the carrier amplifier operates, and on the other hand, the peak amplifier does not operate. However, when the input signal voltage is larger than a pinch-off voltage biased in class C mode of the peak amplifier, the peak amplifier rises up. Then, an output signal from the peak amplifier is synthesized in the same phase as that from the carrier amplifier via a λ/4 line. As can be seen from the above sequence, the Doherty circuit has excellent power efficiency in view of the fact that only the carrier amplifier operates during a small-signal operation.
Note that in the Doherty circuit, since linearity is deformed, intermodulation distortion is caused. To cope with the above-described problem, as a technique for canceling distortion, a distortion compensation is mainly performed by using digital pre-distortion (hereinafter, referred to as DPD).
However, there is a problem that since the gate bias of the Doherty circuit is fixed, when an optimal operating point fluctuates based on a temperature change, the gate bias and the optimal operating point are not matched with each other, and therefore, the power efficiency is deteriorated.
In addition, there is a problem that when the optimal operating point fluctuates based on a frequency change of input signals, the gate bias and the optimal operating point are not matched with each other, and therefore, the power efficiency is deteriorated.